Do you mean quantity, or inventory?
Inventory can be added on the variation's tab under "Stock Qty"
http://docs.woothemes.com/wp-content/uploads/2012/01/product-variations-4.png
Quantity can be added on the product page when adding to the cart.
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Daniel Espinoza on "[Plugin: WooCommerce - excelling eCommerce] How to create product accessories in woo-commerce"
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